Satellite view of iceberg A23a in the Southern Ocean, showing meltwater on its surface
NASA
Meltwater on a city‑sized iceberg in the Southern Ocean is rapidly forming a giant pool on its surface – possibly a sign that it is close to breaking apart.
Scientists are captivated by the frozen colossus, known as A23a, because meltwater is collecting and being held on its surface in an unusual way.
Satellite images reveal a raised rim of ice running around the entire cliff edge of the tabular Antarctic iceberg, giving it the appearance of an oversized children’s play pool — except this one spans about 800 square kilometres, an area larger than Chicago.
In places, the ponded water appears a deep, vivid blue, suggesting depths of several metres. Across the whole of A23a, the water volume probably runs into billions of litres – enough to fill thousands of Olympic‑sized swimming pools.
Douglas MacAyeal at the University of Chicago says the rim effect is typical of the world’s largest icebergs.
“My theory is that the edges are bent, nose‑down, creating an arch‑like dam on the top surface that keeps the meltwater inside,” he says. “The bending is probably a combination of cliff-face undercutting by waves and melting, and the natural tendency for ice cliffs to bend over even if they would be perfectly vertical otherwise.”
The streaks of surface water visible in the satellite imagery are a relic of the way the ice once flowed when the iceberg was still attached to Antarctica’s coastline, he says.
A photo of the iceberg taken by an astronaut on the International Space Station on 27 December 2025
NASA
A23a is an old iceberg. It calved from the Filchner–Ronne ice shelf in 1986 and was then more than five times its current size. For a while, it held the title of the world’s largest iceberg.
In recent years, however, it has drifted north into warmer waters and air, and is now undergoing relentless fragmentation. The sheer volume of meltwater pooling on its surface may finally break it apart. “If that water drains into cracks and refreezes, it will prise the berg open,” says Mike Meredith at the British Antarctic Survey.
It could, he says, turn to mush almost overnight.
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